For any electrical applications, it is necessary to convert power line AC into DC and then to convert this DC back into AC. Examples are switching power inverters, electronic ballasts, etc.
Common circuits for converting the power line AC into DC are half wave rectifier circuits and full wave rectifier circuits, and normally, the rectified voltage and current output from these rectifier circuits consists of a series of unidirectional waves or ripples. For some applications, these variations are not objectionable, but for others they must be smoothed out by filters.
Rectifiers without output filters find limited application owing to their high ripple output and relatively low conversion efficiency. Since the ultimate purpose of rectification is to produce a steady output voltage or current, the pulsating current from a rectifier must be smoothed, and the most common method of smoothing the rectifier output ripples is to use a large storage capacitor which is connected across the rectifier output. However, this storage or filter capacitor creates a potential problem; namely, a high inrush current. Due to the high storage capacity of the filter capacitor and its low equivalent series resistance, the filter capacitor behaves like a nearly perfect short circuit when the power supply first turns on. The resulting short-duration peak inrush current can reach levels much greater than the tolerable single-cycle ratings of conventional semiconductor rectifiers and therefore, inrush current often destroys rectifiers and other electronic components. Such inrush current also generates undesirable current surges in the input power line.
A well-known approach to limit the inrush current in a conversion circuit is by connecting a series resistor between the output of the rectifier and the filter capacitor. This is simple and reliable but not efficient. At any current level, the series resistor dissipates power that would otherwise be available to the load.
Another common method of limiting the inrush current is to use a negative temperature coefficient resistance, such as a thermistor, in place of the series resistor. At turn-on, the "several ohms" resistance in the thermistor limits the inrush current which, in turn, produces power in the thermistor. The power in the termistor raises its temperature, and as a result, the resistance of the thermistor drops to a low value for normal operation. This is a very simple, economical and effective way to limit the inrush current. However, the use of a negative temperature coefficient thermistor for inrush current limiting can be very ineffective in some situations. A power converter may be operating at full load where the thermistor is very low in resistance, and a power interruption could occur which is long enough to cause the discharge of the filter capacitor, but too short for the thermistor to return to a high resistance state. Under this circumstance, when the AC voltage returns, a high inrush current will occur. This is true because the thermal time constant of a negative temperature coefficient thermistor is usually long and, as a result, a short term power interruption may not allow enough time for the thermistor resistance to sufficiently increase.